WO2009009573A2

WO2009009573A2 - System and method for regulation of light emitting diodes

Info

Publication number: WO2009009573A2
Application number: PCT/US2008/069496
Authority: WO
Inventors: Hannes Estl; Sanmukh M. Patel; Rex M. Teggatz; Luthuli E. Dake
Original assignee: Texas Instruments Incorporated
Priority date: 2007-07-09
Filing date: 2008-07-09
Publication date: 2009-01-15
Also published as: WO2009009573A3

Abstract

System and method for regulating light emitting diodes. An embodiment comprises providing a drive current to a light emitting diode (block 320), measuring a forward voltage drop over the light emitting diode (block 355), comparing the forward voltage drop with a threshold (block 360), and altering the drive current in response to a determining that the forward voltage drop is less than the threshold (block 365). Due to a substantially linear relationship between a light emitting diode's forward voltage and its temperature, it is possible to determine if the light emitting diode is overheating by measuring the forward voltage and comparing it with the threshold. The use of the forward voltage eliminates the need for a temperature sensor, thereby reducing system complexity and cost.

Description

SYSTEM AND METHOD FOR REGULATION OF LIGHT EMITTING DIODES

The present invention relates generally to a system and method for operating light emitting diodes, and more particularly to a system and method for light emitting diode regulation.

BACKGROUND

A light emitting diode (LED) is a semiconductor device that emits light when electrically biased in a forward direction with respect to the semiconductor device's junction. In general, the greater the current flowing through the LED, the brighter the light emitted by the LED. However, too large of a current (peak and/or average) may permanently damage or destroy the LED in short order.

In addition to damage from excessive currents, an LED may be damaged if allowed to overheat. As charge carriers cross an LED's junction, heat is produced in addition to light. Excessive heat build-up in an LED may shorten the useful life of an LED. In many cases, overheating may shorten the useful life of an LED by a factor of two or more. Therefore, the operating temperature of an LED should be carefully monitored and regulated. SUMMARY

These and other problems are addressed, and technical advantages achieved, by embodiments of a system and method for light emitting diode regulation. In accordance with an embodiment, a method for illuminating a light emitting diode is provided. The method includes providing a drive current to the light emitting diode, measuring a forward voltage drop over the light emitting diode, and comparing the forward voltage drop with a first threshold. The method also includes altering the drive current in response to a determining that the forward voltage drop differs from the threshold by more than a second threshold.

In accordance with another embodiment, a method for operating a light source is provided. The method includes determining a set of first thresholds and a set of second thresholds, storing the set of first thresholds and the set of second thresholds, selecting a first threshold and a second threshold, and providing a drive current to a light emitting diode of the light source. The method also includes regulating an operating temperature of the light emitting diode. The regulating includes measuring a forward voltage drop over the light emitting diode, and comparing the forward voltage drop with a first threshold.

In accordance with another embodiment, a light source is provided. The light source includes a light emitting diode to produce light, and a light controller circuit coupled to the light emitting diode. The light controller circuit measures a forward voltage drop over the light emitting diode and alters a drive current provided to the light emitting diode to change an operating temperature of the light emitting diode if the forward voltage drop differs from a first threshold by more than a second threshold.

An advantage of an embodiment is that the operating temperature of an LED may be monitored and regulated without requiring the inclusion of hardware to measure the temperature of the LED. This may help to reduce the cost of LED driver hardware as well as reducing the overall size of the driver hardware.

A further advantage of an embodiment is that the operating temperature of an LED may be monitored directly at the LED, providing a more accurate indicator of LED operating temperature. The use of typical direct temperature measurement hardware may be limited to measuring the temperature of the packaging of the LED, for example, precluding a precise measurement of the temperature of the LED itself. BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments of the claimed invention are described with reference to accompanying drawings, wherein:

FIG. 1 is a data plot of forward voltage versus operating temperature;

FIG. 2 is a diagram of a light source;

FIGS. 3 A and 3B are diagrams of sequences of events in operating a light source and regulating operating temperature; FIGS. 4A-4D are diagrams of pulse- width modulated drive currents; and

FIG. 5 is a diagram of a constant drive current. DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Example embodiments are described in a specific context, namely a circuit for providing current to illuminate a light emitting diode. The circuit and the light emitting diode may be a part of a light source, such as for a projection image display system including those utilizing microdisplays such as digital micromirror devices, transmissive or reflective liquid crystal displays, liquid crystal on silicon, ferroelectric liquid-crystal-on-silicon, deformable micromirrors and so forth, automotive lighting, a flashlight, household lighting, factory lighting, and so forth. The invention may also be applied, however, to other circuits used to provide current to diodes, optoelectric devices, and other semiconductor devices, wherein a current flowing through the device's junction may cause a heating of the junction.

FIG. 1 shows a data plot illustrating forward voltage versus operating temperature for an example LED. Three data curves are shown, each curve representing a different value for drive current. A first curve 105 displays the forward voltage versus operating temperature for a drive current of about 20 mA, a second curve 110 displays the forward voltage versus operating temperature for a drive current of about 50 mA, and a third curve 115 displays the forward voltage versus operating temperature for a drive current of about 100 mA. The three curves indicate that there may be a substantially linear relationship between the forward voltage of the LED and the LED's operating temperature for a constant drive current. Furthermore, the relationship may also be described as being strictly decreasing or monotonic. Exact values of forward voltage and operating temperature may differ for different LEDs based on the materials used in the LEDs construction as well as their manufacturing process. However, the linear relationship between forward voltage and operating temperature generally remains consistent across various types of LEDs as well as other semiconductor devices. The substantially linear relationship between an LED's forward voltage and its operating temperature may be used to regulate the temperature of LEDs and protect the LED from overheating.

For example, a constant current may be used to drive an LED and then the LED's forward voltage may be measured and used to protect and regulate the temperature of the LED. It may also be possible to provide a constant voltage to an LED and measure the forward current to protect and regulate the temperature of the LED.

FIG. 2 illustrates an example light source 200. The light source 200 includes an LED 205 and a light controller circuit 210. The LED 205 may include a number of LEDs, with the number dependent upon a desired amount of light, the light emitting capabilities of each individual LED, the number of colors desired, and so on. The light controller circuit 210 may be used to provide a drive current to the LED 205 as well as provide temperature detection, regulation, and protection for the LED 205. The light controller circuit 210 includes a controller 215, a light driver circuit 220, a memory 225, and a voltage sensor 230. The controller 215 may provide commands to the light driver circuit 220, which may produce the drive current that may be used to drive the LED 205. For example, the controller 215 may provide a command to the light driver circuit 220 to provide a drive current at a given magnitude, duty cycle, and so forth. Alternatively, the controller 215 may provide a simple on/off command that may be used to turn the drive current from the light driver circuit 220 on or off. The memory 225 may be used to store a variety of different drive currents that may be provided to the LED 205 to create light of differing brightness, color point, and so forth. The voltage sensor 230 may be used to measure the forward voltage of the LED 205. The measured forward voltage may then be provided to the controller 215. The controller 215 may compare the measured forward voltage with a threshold value to determine if the LED 205 might be overheating. If the measured forward voltage is less than the threshold, then the LED 205 may be overheating. According to a preferred embodiment, the measured forward voltage may be strictly less than the threshold or the measured forward voltage may be less than the threshold by a delta before a determination is made that the LED 205 may be overheating. In general, the requirement of the measured forward voltage being less than the threshold by a delta may accommodate differences in LED forward voltages due to manufacturing process variations, for example.

When the LED 205 has been determined to be overheating, the controller 215 may help to cool the LED 205 in a number of different ways, such as by reducing the duty cycle of the drive current driving the LED 205, reducing the peak current of the drive current driving the LED 205, reducing the average current of the drive current driving the LED 205, increasing the operation of a cooling system used to cool the LED 205, and so forth, or any combination thereof.

The memory 225 may be used to store multiple thresholds and deltas to enable support for a wide variety of drive current duty cycles and magnitudes as well as a variety of different LEDs. This may enable the use of a single design of the light controller circuit 210 with multiple different LEDs from different manufacturers and manufacturing processes. The multiple thresholds and deltas may be stored in the memory 225 during the manufacture of the light controller circuit 210. Alternatively, the multiple thresholds and deltas may be stored in the memory 225 when the LED 205 is installed with the light controller circuit 210 during the manufacture of the light source 200, such as during a testing phase of the manufacture of the light source 200. For example, the LED 205 may be illuminated and various drive current values may be used to derive the multiple thresholds and deltas for the particular LED used in the light source 200. This may provide compensation for differences between substantially identical LEDs that are manufactured in the same manufacturing process, but may function slightly differently due to manufacturing process variations, for example. In addition to storing the thresholds and deltas for use in LED temperature regulation and protection, the memory 225 may also be used to store system calibration and configuration data.

FIG. 3A illustrates a sequence of events 300 in the operation of an LED-based light source with LED temperature detection and protection, such as the light source 200. The sequence of events 300 may begin with a determination of thresholds and deltas (block 305). The thresholds and deltas may be dependent upon the drive current's magnitude and duty cycle, as well as the characteristics of the LED used in the LED-based light source. Additionally, the presence of a cooling system may have an impact on the values of the thresholds and the deltas. Examples of LED characteristics may include the materials used in the fabrication of the LED, the manufacturing process and manufacturing process variations, and so forth.

The thresholds and deltas may then be stored in a memory, such as the memory 225 (block 310). The determination of the thresholds and the deltas, as well as storing the thresholds and the deltas in a memory (blocks 305 and 310) may be performed prior to the light source 200 being placed into use. For example, a manufacturer of the light source 200 or the light controller circuit 210 may store, in the memory 225, a number of thresholds and deltas for a variety of LEDs during the manufacture or testing of the light source 200 or the light controller circuit 210. Alternatively, a manufacturer of a system containing the light source 200 may store in the memory 225 a number of thresholds and deltas for a particular LED chosen by the manufacturer during the manufacture of the system. After the thresholds and the deltas have been determined, a single threshold and delta may be selected (block 315). The selected threshold and delta may be chosen based on characteristics of an LED, such as the LED 205, used in the light source 200, as well as drive current, desired brightness, chromatic characteristics, and so forth. With the selection of the threshold and the delta, the drive current may be provided to the LED 205 to cause the LED 205 to emit light (block 320). As the LED 205 is emitting light, the temperature of the LED 205 may be continuously regulated (block 325). This may be achieved by measuring and changing the temperature as the drive current is being provided to the LED 205. The providing of the drive current to the LED 205 along with the regulation of the LED's temperature may be continued until the light source 200 is powered down.

FIG. 3B illustrates a sequence of events 350 providing a detailed view of the providing of the drive current to the LED 205 and the regulation of the LED's temperature (blocks 320 and 325 of the sequence of events 300). After the drive current has been provided to the LED 205 (block 320), the LED's forward voltage may be measured (block 355). The forward voltage of the LED 205 may be measured with a voltage sensor, such as the voltage sensor 230. Since there is a substantially linear relationship between the forward voltage of the LED 205 and the LED's temperature, the measured forward voltage may be used to indicate the temperature of the LED 205.

The forward voltage of the LED 205 may be measured at several different instances of time with respect to the drive current being provided to the LED 205. For example, the drive current may comprise two distinct periods, a first period wherein a current with a desired magnitude may be provided to the LED 205 and a second period wherein no current (or substantially no current) may be provided to the LED 205. The forward voltage of the LED 205 may be measured just as the first period begins, a specified amount of time after the first period begins but before the first period ends, just as the first period ends, a specified amount of time after the first period ends but before the second period ends, and so forth.

The measured forward voltage of the LED 205 may then be compared with the selected threshold (block 360). If the measured forward voltage is less than the selected threshold then the LED 205 may be overheating. According to an embodiment, the measured forward voltage may be strictly less than the selected threshold or the measured forward voltage may be less than the selected threshold by more than the delta before the LED 205 may be considered to be overheating. The use of the delta in the comparison may provide a measure of desensitizing of the comparison to variations in the forward voltage of LEDs due to manufacturing process variations. If the measured forward voltage is not less than the selected threshold, then the LED is determined to not be overheating and the sequence of events 350 may repeat. Potentially, since the LED may not be overheating, the drive current may be increased (the drive current's peak current and/or average current may be increased) if additional light is required.

If the measured forward voltage is less than the selected threshold, then the LED may be overheating and the drive current may be altered to reduce the LED's temperature (block 365). The drive current may be altered to change the drive current's peak current, average current, or both peak current and average current. Since the threshold and delta may be based on the drive current, after altering the drive current, it may be necessary to reselect the threshold and delta based on the altered drive current. After altering the drive current and reselecting the threshold and delta, the sequence of events 350 may continue.

FIGS. 4A - 4D illustrate example pulse- width modulated (PWM) drive currents. FIG. 4A illustrates an example PWM drive current (shown as curve 405). The PWM drive current includes a series of current pulses, such as pulse 407, between periods of no-current pulses, such as pulse 409. Although referred to as being no-current pulses, the no-current pulses may have a small measurable current, with the current being substantially smaller in magnitude than the magnitude of the current of the current pulses. The no-current pulse 409 may have a duration (shown as duration 410) and the pulse 407 may have a duration (shown as duration 412). The pulse 407 may also have a magnitude (shown as magnitude 414). Generally, the pulse 407 and the no-current pulse 409 are repeatedly provided to the LED 205 until there is a need to change the light emission from the LED 205.

FIG. 4B illustrates a modified PWM drive current, shown as curve 425, wherein the duration of a current pulse, such as pulse 427, has been modified. The duration of the pulse 427 has been shortened (shown as duration 430). Therefore, with the period of the pulses maintained, the duration of no-current pulses (shown as duration 432) has been extended. The shortening of the pulse 427 may result in a reduction in the average current of the PWM drive current and may result in the LED 205 emitting less light, thereby not producing as much heat. This may potentially prevent overheating. Alternatively, the period of the pulses is shortened and the duration of the no-current pulses is not altered. In yet another embodiment, the duration of current pulse 427 is maintained while the duration of no-current pulses may be extended.

FIG. 4C illustrates a modified PWM drive current, shown as curve 445, wherein the magnitude of a current pulse, such as pulse 447, has been modified by being reduced (shown as magnitude 450). The adjustment to the magnitude of the pulse 447 may not have an impact on the period of the pulses. The adjustment to the magnitude of the pulse 447 may result in a reduction in the peak current of the drive current and potentially a reduction in the average current of the drive current (unless the period of the pulses are also adjusted). The reduction in the magnitude of the pulse 447 may result in the LED 205 emitting less light, thereby not producing as much heat. This may potentially prevent overheating.

FIG. 4D illustrates a modified PWM drive current, shown as curve 465, wherein the shape of a current pulse, such as pulse 467, has been modified. The shape of the pulse 467 may have been modified so that it now has the appearance of a triangle. Altering the shape of the pulse 467 may change both the peak current and/or the average current of the drive current. Although the shape of the pulse 467 is shown in FIG. 4d as having a triangular shape, it may be possible to modify the shape of a pulse to any of a wide variety of shapes, including but not limited to semicircular, sinusoidal, irregular-multifaceted pulse, and so forth.

FIGS. 4A - 4D illustrate only some of many techniques for adjusting a PWM drive current. Many other modifications to a PWM drive current may be possible, including combining several techniques, such as altering both the magnitude and the duration of a pulse. Therefore, the discussion of specific techniques should not be construed as being limiting to either the scope or the spirit of the embodiments.

In addition to a PWM drive current, it may be possible to drive an LED with a continuous current, such as a current shown as curve 505 in FIG. 5. The current has a nonzero magnitude (a dashed line 510 indicates zero current). In a PWM drive current, the drive current's peak current and average current may differ, while with a continuous current, the drive current's peak and average current may be identical. In this case, the controller may prevent overheating by reducing current.

Those skilled in the art will appreciate that other embodiments and variations are possible within the scope of the claimed invention; and also that embodiments having different combinations of one or more features or steps are intended to be covered hereby even though for brevity or simplicity those features or steps are described in the context of example embodiments having all or just some of such features or steps.

Claims

CLAIMSWhat is claimed is:

1. A method for illuminating a light emitting diode, the method comprising: providing a drive current to the light emitting diode; measuring a forward voltage drop across the light emitting diode; comparing the forward voltage drop with a first threshold; and altering the drive current in response to determining that the forward voltage drop differs from the first threshold.

2. The method of Claim 1, wherein the altering is performed if the forward voltage drop is less than the first threshold by more than a second threshold.

3. The method of Claim 1 or 2, wherein the drive current comprises a sequence of current pulses, and wherein the measuring occurs a specified amount of time after each current pulse is provided to the light emitting diode.

4. The method of Claim 3, wherein the measuring completes before each current pulse completes.

5. The method of Claim 1, wherein the drive current comprises one or more current pulses, and wherein the altering is selected from the group consisting of: a) reducing a magnitude of a current pulse; b) shortening a duration of a current pulse; c) extending a duration between consecutive current pulses; d) changing a shape of a current pulse; and e) combinations of any of a) through d).

6. A method for operating a light source, the method comprising: determining a set of first thresholds; storing the set of first thresholds; selecting a first threshold; providing a drive current to a light emitting diode of the light source; and regulating an operating temperature of the light emitting diode, wherein the regulating comprises, measuring a forward voltage drop over the light emitting diode; and comparing the forward voltage drop with the first threshold.

7. The method of Claim 6, wherein the determining further comprises determining a set of second thresholds; the storing further comprises storing the set of second thresholds; the selecting further comprises selecting a set of second thresholds; and the comparing comprises comparing the forward voltage drop with the first threshold and the second threshold.

8. The method of Claim 7, wherein the regulating further comprises, after the comparing, altering the drive current in response to determining that the forward voltage drop differs from the first threshold by more than the second threshold.

9. The method of Claim 6, further comprising, after the comparing: altering the drive current in response to a determining that the forward voltage drop is less than the first threshold; and reselecting a new first threshold and a new second threshold based on the altered drive current.

10. The method of any of Claims 6 through 9, wherein the first threshold is selected based on the drive current, a desired brightness of the light emitting diode, and device characteristics of the light emitting diode.

11. A light source comprising: a light emitting diode to produce light; and a light controller circuit coupled to the light emitting diode, the light controller circuit configured to measure a forward voltage drop over the light emitting diode and to alter a drive current provided to the light emitting diode to change an operating temperature of the light emitting diode if the forward voltage drop is less than a threshold.

12. The light source of Claim 11, wherein the light controller circuit comprises: a sensor coupled to the light emitting diode, the sensor to measure the forward voltage drop; and a controller coupled to the sensor, the controller configured to alter the drive current if the forward voltage drop is less than the threshold.

13. The light source of Claim 11 or 12, further comprising a memory coupled to the controller, the memory to store the threshold.

14. The light source of Claim 13, wherein the memory stores a set of thresholds, wherein the threshold may differ in value based on different drive currents, desired light emitting diode light intensity, light emitting diode material characteristics, and combinations thereof.

15. The light source of Claim 11 or 12, further comprising a light driver circuit coupled to the controller and to the light emitting diode, the light driver circuit to provide the drive current to the light emitting diode based on control instructions from the controller.